Implementation of a sectional aerosol package with comprehensive SOA formation into CAM5

 

Authors

Catherine Chuang — Lawrence Livermore National Laboratory
Arthur Mirin — Lawrence Livermore National Laboratory
Dan Bergmann — Lawrence Livermore National Laboratory
Philip Cameron-Smith — Lawrence Livermore National Laboratory

Category

Aerosol Properties

Description

Properties of atmospheric aerosols and aerosol-cloud interactions depend strongly on aerosol size distribution, composition, and mixing state. These microphysical characteristics underlie the major role of aerosols in radiative forcing of climate. However, both existing chemistry-aerosol modules (i.e., “bulk” and “modal”) in CAM5 do not resolve the detailed aerosol size distribution, and they oversimplify the formation of secondary organic aerosols (SOAs) that make up a significant portion of global aerosol burden.

To address these issues, we implemented a sectional aerosol package into CAM5 (CAM5/Sect) that integrated the MOZART (Model of Ozone and Related Tracers) gas chemistry with MADRID (Model of Aerosol Dynamics, Reaction, Ionization, and Dissolution) aerosol microphysics to precisely link precursor gases to aerosol formation and size distribution. The new chemistry-aerosol package contains a total of 210 reactions, including 10 reactions for SOA chemistry to produce condensable semi-volatile organic gas species. Formation of SOAs is simulated from 4 anthropogenic and 14 biogenic organic aerosol precursors.

We modified the land model in CAM5 to enable the interactive biogenic emissions of 15 volatile organic carbon (VOC) compounds, which is essential for the representation of tropospheric chemistry. Emissions of VOCs are based on an online biogenic emission system, MEGAN (Model of Emissions of Gases and Aerosols from Nature). In additional to SOAs, other aerosol types simulated include sodium, sulfate, ammonium, nitrate, chloride, dust, primary black carbon, and organic carbon. These prognostic size-resolved aerosols are subsequently coupled to photolysis, heterogeneous, and aqueous phase chemical reactions. A new treatment for aerosol/cloud interactions has been introduced into CAM5/Sect to account for the dependency of droplet formation on aerosol size and composition in each sectional bin.

We started with 2-degree resolution and 8 size bins (0.02–10 micron in diameter) and are gradually moving toward our target resolution of 0.5-degree and 16 bins for intensive periods during field campaigns. The number of constituents carried in CAM5/Sect is 335, as compared to 25 for the standard version and 105 for MOZART. Preliminary results from CAM5/Sect will be presented.